Critical speed occurs when an object’s rotational motion matches one of its natural frequencies, causing resonance and high amplitude vibrations. This is studied in rotor dynamics, and is important for designing rotating objects to avoid damage. The first critical speed is the lowest frequency causing resonance, and passing through critical speeds quickly is crucial for the longevity of the machine.
An object is said to reach critical speed when the speed of its rotation matches one of its natural frequencies. This type of velocity is studied in a branch of physics known as rotor dynamics, which deals with rotational or angular motion. A rotating object, such as a propeller or centrifugal pump, often has to pass through one or more of its critical speeds as it accelerates or decelerates. When operating at critical speed, these objects vibrate at a high amplitude, which can cause damage.
All objects that are composed of an elastic material have one or more natural frequencies. The natural frequency of an object is the number of times it will move back and forth once it has been set in motion. When an object vibrates at one of its natural frequencies, it is said to have resonance, or a large vibrational amplitude. In a musical instrument, for example, this resonance is desirable because it causes a natural amplification of the instrument’s sound. In rotor dynamics, on the other hand, this resonance is undesirable because it causes the mechanical parts involved to vibrate strongly, which can damage the system.
There are a number of stimuli that can cause resonance, one of which is rotational motion. When an object’s rotational motion, also called angular velocity, causes resonance, it is at critical velocity. Rotating mechanical objects must be designed to pass through these speeds quickly so that the amplified vibration that occurs at this speed does not cause damage.
A centrifugal pump or propeller will move through different angular speeds as it accelerates or decelerates. While a moving system such as this will naturally have vibration, vibration amplified at the critical speed must be avoided or traversed quickly if the system is to stand the test of time. Rotordynamics is therefore very interested in the resolution of the various critical speeds that can affect the life of a rotating machine.
The lowest rotational frequency that causes an object to vibrate at one of its natural frequencies is known as the first critical speed. An object can have an infinite number of critical speeds, but the most important for engineers to consider is the first. Some rotating objects are designed to operate below their first critical speed, but many rotate above this speed. As long as the rotational speed passes rapidly through the critical speed, this should not have a significant effect on the life of the rotating machine. However, problems can occur if an object is designed to rotate at its critical speed, as the resulting vibrations could cause the machine to break down.
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